Predicting pore pressure in active fold–thrust systems: An empirical model for the deepwater Sabah foldbelt

• Published in: Journal of structural geology 2014-12, Vol.69, p.465-480
• Main Authors: Couzens-Schultz, Brent A., Azbel, Konstantin
• Format: Article
• Language: English
• Subjects: Acoustic velocity; Belts; Compacts; Compressive properties; Empirical analysis; Mathematical models; Mudrock properties; Pore pressure; Porosity; Stress; Stresses; Thrust belt; Trends
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2014.07.013

Measurements related to mudrock (shale and siltstone) porosity such as acoustic velocity, density or electrical resistivity, have traditionally been used to predict pore pressures in extensional stress settings. The underlying assumption is that burial and vertical effective stress (VES), which is the overburden minus the pore pressure, controls the compaction of these rocks through porosity loss. The dataset presented here compares VES and acoustic velocity of similar composition mudrocks in both an extensional and a compressional stress setting. In the extensional stress environment, the mudrocks follow a typical compaction trend with a porosity loss and increase in acoustic velocity that can be related to VES. In an active fold–thrust belt, the compressive stresses further reduce the porosity and increase the acoustic velocity of the mudrocks. First a layer-parallel shortening compacts sediments beyond what is observed for the VES. This additional compaction is further enhanced near thrust faults and in anticlinal forelimbs, presumably due to additional shear stress in these areas. The mudrocks located in folds that are buried by additional sedimentation do not compact again until the tectonic compaction is overridden by enough new burial. After that, the mudrocks follow the observed extensional setting compaction trend. In the fold–thrust belt, the observed reduction in porosity by stresses other than burial leads to an under-prediction of pore pressure using traditional methods. To account for this, we present a correction that can be applied to the acoustic velocity (or porosity) using two parameters: (a) proximity to thrust faults and anticlinal forelimbs and (b) the amount of burial after fold formation. With these corrections, the extensional velocity–VES compaction trend can be used to accurately predict pore pressure within the active fold–thrust belt. The correction is calibrated with well data and is empirical. None-the-less, it is a first step toward understanding the magnitudes of the compressive lateral stresses involved in generating the observed porosity loss within the fold–thrust belt. •Lateral stresses impact pore pressure and mudrock properties.•The largest impact is seen in the steep forelimb and anticlinal core of folds.•Structural position is used as a proxy for the impact of lateral stresses on porosity loss.•A method is presented that corrects porosity-based measurements for lateral stress effects to predict pore pressure.